In the movie called The Crucible the church and court both state all liars will be damned, but only God can tell who the real sinners are. What does this mean? What are they trying to say ?

The movie of The Crucible is a film version of a play by Arthur Miller. I should note that close analysis is much easier if you read the text of the play.
Miller wrote the play as a critique of the ideologically grounded persecutions of people suspected of being communists or sympathizing with them in the United States in the 1950s. He equates religion with other forms of persecution grounded in narrow-minded ideological certainties. 
Miller's argument here is that human beings should not attempt to enforce divine justice because they lack divine omniscience. In other words, within Christian theology, it is believed that God knows people's innermost hearts and thus can judge them on the basis of their ideas and feelings as well as actions. Human beings on the other hand, whether affiliated with state or church, lack such knowledge and thus cannot and should not condemn people on ideological grounds.
For Miller, both church and state could condemn someone for specific acts such as murder or assault, but not for heresy or other beliefs as questions of belief involve understanding people's souls, something that God may be able to do but humans can't manage.